This invention relates to a process for the batch-wise filtering of sludge-like materials in a filter press.
A similar process is disclosed in German patent DE-PS No. 1,461,452. An advantage is achieved by feeding sludge into a pressure vessel containing air and connected to a filter press in that the loading of the filter press at the commencement of each batch can be achieved very quickly by removing the sludge from the pressure vessel by means of the compressed air therein. In addition, a pump can also feed sludge into the filter press. After the initial rapid loading, during which the air pressure in the pressure vessels declines, the actual filtration process is carried out by further conveying of sludge into the filter press by the pump. The pump thereby causes pressure to increase continually in the filter press until finally a pressure limit is achieved and the filtration phase is completed.
The pressure vessel, partly emptied by the loading process, must be refilled by the conveying of sludge thereto before each next loading step. In the case of conventional processes this is carried out during the actual filtration phase in such a way that a part of the sludge conveyed by the pump into the filter press is again removed at a point distant from the feed point and passed into a pressure vessel containing air. An advantage arising out of this, lies in the fact that a strong flow is maintained during the filtration step which thus prevents the flow orifices in the filter press from becoming blocked. However, the conventional process has a disadvantage in that during the filtration phase only a part of the sludge conveyed by the pump is used for the filtration while another unused part is returned and again removed. This means that at a given pump capacity the filtration step is extended or the quantity of filtered sludge is reduced. Accordingly, the pump capacity is not used to the optimum degree.
Other known processes operating with pressure vessels containing air also have essentially the same disadvantages. However, they do not belong to the category mentioned above since in their case the pressure vessel is not used for the rapid loading of the filter press, but for the actual filtration process. According to German patent application No. DE-OS 1,536,874 two pumps are used, namely a low pressure pump for the initial loading of the filter press, and a high pressure pump for the concurrent filling of the pressure vessel containing air. During the actual filtration process both pumps are switched off and the filter press is fed with sludge from the pressure vessel containing air. In the case of the process according to German patent application No. DE-OS 2,530,550 the pump loads the filter press and also conveys further sludge during an initial stage of the filtration step, while at the same time sludge is removed again from the filter press and stored in the pressure vessel containing air. However, the sludge stored is not passed to the filter press at the next loading phase, since the sludge has already passed the filter press during the second stage of the filtration step. Accordingly, the pump is switched off immediately when a predetermined pressure limit is reached and filtration is continued under falling pressure with sludge flowing from the pressure vessel containing air. It will be noted that with both processes the pumps are kept in operation during only a relatively small part of the total filtration batch and thus are not used to the optimum degree.
Prior art systems having a pressure vessel communicating with a filter press are also shown in our prior U.S. Pat. Nos. 3,502,210 and 4,067,807.